1. Field of the Invention
The present invention relates to the production of a partially pyrolyzed urea composition particularly suitable for use as the feedstock for the production of animal feed grade biuret, and containing controlled amounts of biuret, cyanuric acid, triuret, and other homologs. Production of such composition involves the controlled pyrolysis of urea at elevated temperatures above the melting point of urea and at a pressure above atmospheric while subjected to sparged air or other non-reactive gas at the rate of about two to about ten cubic feet of gas per hour pound of urea, and cooling and comminuting the resulting product.
2. Description of the Prior Art
Harmon U.S. Pat. No. 2,145,392 discloses a basic process for pyrolysis of urea wherein urea is heated in the temperature range of 130.degree.-205.degree. C. while under partial vacuum in the absence of any catalyst to produce principally biuret, cyanuric acid, and other related compounds. Biuret is a useful chemical compound having considerable utility in prepared feeds for ruminant animals. Crude technical mixtures of urea, biuret, triuret, and cyanuric acid have long been used as cattle feed supplements. The United States Department of Agriculture, Food and Drug Administration has approved pyrolyzed urea compositions having not more than 15% urea, not less than 55% biuret, and not more than 30% of the sum of cyanuric acid, triuret, or other related compounds, and not more than 0.5% of oil, by weight, as a cattle feed additive.
Garbo U.S. Pat. No. 2,525,049 discloses much the same process for producing biuret from urea as is disclosed by Harmon U.S. Pat. No. 2,145,392, with the added feature of accelerating the reaction by the use of one or more catalysts and in some instances use also of a hydrocarbon fluidizing medium such as naphthalene or kerosene.
Olin U.S. Pat. No., 2,370,065 presents another teaching of use of an entraining agent such as toluene or naphtha to aid in the removal of ammonia when pyrolyzing urea to produce biuret.
Kluge U.S. Pat. No. 3,150,177 presents a biuret production process similar to that disclosed by Harmon U.S. Pat. No. 2,145,392 coupled with the use of disodium phosphate or boric acid as a catalyst.
Kamlet U.S. Pat. No. 2,768,895 presents a rather omnibus disclosure of use of biuret rich urea condensation products as animal feed. Kamlet U.S. Pat. No. 3,453,098 presents a variation of the process wherein biuret rich urea autocondensation products are dissolved in boiling water and then cooled to recrystallize the biuret products, which are then said to contain not less than 95% biuret and be characterized by complete removal of ammonium cyanate, the product thus being rendered suitable for use as a depot fertilizer without the phytotoxic consequences said to be characteristic of such a product when substantial ammonium cyanate is present.
Colby et al, U.S. Pat. No. 2,861,886 presents another early disclosure of use of biuret rich compositions used as constituent ingredients of animal feed supplements.
Great Britain Pat. No. 1,155,907 discloses the pyrolysis of urea to product a biuret rich reaction product, including the separation of a urea containing mixture from the reaction product, and recycling of the separated urea containing mixture to the reactor as a continuous process.
Japan Patent Publication No. 47-41888 (1972) discloses the production of purified biuret. The English language abstract published with Japan Patent Publication No. 47-41888 states that this publication discloses a process for preparing purified biuret by treating urea decomposition products with hot water, filtering the obtained mixture, treating the filtrate with aqueous ammonia and cooling it to below 40.degree. C., separating the precipitated biuret, recovering ammonia from the supernatant liquor by evaporating water, and recycling residual urea after decomposing it by heating. Substantially pure biuret (separated from cyanuric acid and triuret) is obtained. Such abstract continues to give the following example. 100 grams urea is decomposed at 150.degree. C. while introducing 22.0 l./min. air for 2 hours to give 38.8 grams biuret, 4.0 grams cyanuric aid, 3.8 grams triuret and 44.0 grams unreacted urea. The products are partly dissolved in 150 ml. water at 70.degree. C. The mixture is filtered and the filtrate is treated with 5 grams NH.sub.3 and cooled to 40.degree. C. The precipitate (24.1 grams; yield 98.8%) is biuret containing 1.2% urea and only traces of cyanuric acid and triuret.
Formaini et al in U.S. Pat. No. 3,057,918 proposes a continuous process for the production of biuret wherein new urea plus recycled previously pyrolyzed urea is sparged with air while molten at elevated temperature to produce a feedstock for a pressure digestion step whereby a relatively pure biuret is separated from the mother liquor of the ammonia digestion step by a controlled vacuum crystallization and the unwanted byproduct is evaporated to give the solid recycle material added along with urea to make the feedstock for the initial pyrolysis step. The urea content of the typical feedstock entering the ammonia digestion step has the following analysis:
______________________________________ Biuret 28-42% Urea 65-40% Cyanuric Acid 3-14% Triuret 5-7% ______________________________________
This is to be contrasted with the typical analysis for the feedstock of the present invention which is:
______________________________________ Biuret 45-60% Urea 37-25% Cyanuric Acid 3-20% Triuret 3-10% ______________________________________
The pyrolysis of urea to produce biuret, triuret, cyanuric acid, ammelide, melamine and other homologs, is well known. When urea is pyrolyzed ammonia is always an accompanying byproduct. It was early established by Harmon in U.S. Pat. No. 2,145,392 that the pyrolysis of urea to produce biuret was aided by the application of controlled vacuum whereby the byproduct ammonia was effectively removed from the reactor thus lowering the partial pressure of ammonia and thereby driving the pyrolysis reaction toward higher biuret content. The use of volatile entraining agents such as toluene to sweep out byproduct ammonia in biuret manufacture was early suggested by Olin in U.S. Pat. No. 2,370,065. The use of gases such as nitrogen or air to sweep out byproduct ammonia formed during urea pyrolysis is suggested in U.S. Pat. No. 2,918,467 by Hibbitts et al in the production of a pyrolyzed urea feedstock used in the production of melamine. Formaini U.S. Pat. No. 3,093,941 uses air as the stripping gas to remove byproduct ammonia formed during urea pyrolysis in the production of a pyrolyzed urea feedstock used in the production of cyanuric acid. Formaini U.S. Pat. No. 3,057,918 uses air as the stripping gas to remove byproduct ammonia formed during continuous urea pyrolysis in the production of a pyrolyzed urea feedstock used in an extraction process to produce substantially pure biuret.